1. Field of the Invention
The invention relates to a burned-gas passage amount computing method and system, used in an exhaust gas recirculation system, that compute the amount of burned gas contained in recirculated exhaust gas.
2. Description of the Related Art
For the purpose of reducing fuel consumption and emission, it is widely practiced to provide an internal combustion engine with an exhaust gas recirculation (EGR) system for recirculating exhaust gas in an exhaust passage of the internal combustion engine into an intake passage thereof. The EGR system includes an EGR passage that connects an intake passage and an exhaust passage and an EGR valve that is provided in the EGR passage and changes the cross section of the EGR passage. By adjusting the degree of opening of the EGR valve according to the engine operating conditions (engine load and engine rotation speed, for example), the amount of gas (EGR gas) returned from the exhaust passage to the intake passage through the EGR system is regulated to the amount appropriate in view of the engine operating conditions.
On the other hand, in the control of operation of an internal combustion engine, so-called fuel cut control is performed, in which fuel supply is temporarily stopped during an operating condition (during deceleration, for example) in which there is no need to output torque. While the fuel cut control is performed, intake air (oxygen-rich air) is discharged into the exhaust passage, so that a condition is created in which there is little amount of burned gas (oxygen-poor gas formed after combustion). Thus, when the fuel cut control is stopped while the EGR valve is open, although temporarily, an air-rich EGR gas is recirculated into the intake passage through the EGR passage. Thus, the amount of air taken into the engine combustion chamber increases and the air-fuel ratio of a mixture becomes unnecessarily lean, so that the operation of the engine can become unstable.
Systems have already been proposed that estimate the ratio of burned gas (burned gas ratio) contained in the EGR gas based on the detection signal from an oxygen concentration sensor for detecting the concentration of oxygen in exhaust gas and perform engine control according to the burned gas ratio thus estimated (see Japanese Patent Application Publications No. 11-236857 and 8-61112 (JP-A-11-236857, JP-A-8-61112), for example). With such a system, the occurrence of a situation is suppressed in which the air-fuel ratio of the mixture becomes lean due to the air contained in the EGR gas, so that it becomes possible to suppress the unstabilization of the operation of the engine.
The concentration of oxygen in the exhaust gas is very high while fuel cut control is performed. However, there is a limit to the concentration of oxygen in exhaust gas that the oxygen concentration sensor can detect. For this reason, when the burned gas ratio is determined based on the signal from the oxygen concentration sensor at each time point, the oxygen concentration cannot be accurately detected and the burned gas ratio cannot be correctly determined if the concentration of oxygen in the exhaust gas goes beyond the detectable range of the oxygen concentration sensor immediately after the fuel cut control is stopped.
Thus, for the purpose of avoiding the unstabilization of operation of the engine due to the influence of the unburned gas contained in the EGR gas, there is a room for improvement even in the case of the above-described system in which the burned gas ratio is determined, based on the signal from the oxygen concentration sensor.